Enclosed switch and fuse units



Oct. 9, 1956 w. s. EDSALL ENCLOSED SWITCH AND FUSE UNITS 2 Shets-Sheet 1 Filed Jan. 28, 1955 Oct. 9, 1956 w. s. EDSALL ENCLOSED SWITCH AND FUSE UNITS 2 Sheets-Sheet 2 Filed Jan. 28, 1955 s. Emilia,

flimrey United States Patent ENCLOSED SWITCH AND FUSE UNITS William S. Edsall, Boston, Mass.,

assignor to The Chase- Shawmut Company,

This invention relates to switch and protective units comprising a non-automatic manually operable switch .and a fusible protective device adapted to be serially con- ;nected into an electric circuit.

Such combinations of switches and of fusible protective devices are frequently applied where current-carryfing capacity requirements and interrupting capacity requirements are relatively high, and where occurrence of faults is relatively infrequent. In such combinations the switch is used to interrupt load currents and to close the circuit, and the fusible protective device to interrupt major fault currents and protracted overload currents.

Generally the switch and the fusible protective device or devices are housed in a common enclosure, such a steel cabinet. Hence the switch is subjected to the considerable heat generated in the fusible protective device -or deviccs. This involves the danger of impairment of the switch by excessive temperatures. As a general rule, the switch must be drastically derated to minimize this danger.

It is, therefore, one object of this invention to provide a combination comprising a non-automatic manually operable switch and fusible protective means for major fault current protection and for overload protection which combination can safely be arranged in a common enclosure without endangering the switch and without need for any derating thereof.

Another related object of this invention is to provide a combination of the aforementioned kind wherein heat generation by the fusible protective means is minimized and wherein heat generation is considerably smaller than in comparable prior art switch-fuse-combinations.

In prior art switch-fuse-combinations overload protection is generally provided by thermally responsive means which operate normally at temperatures considerably higher than the danger temperatures of organic insulating materials, i. e. the temperatures which, if maintained :sufiiciently long, would tend to seriously shorten the life .of the insulating materials. If such a thermally respon- :sive means, e. g. a fuse, is arranged in series with and adjacent to a switch whose contacts are defective and generate unduly large amounts of heat, the thermally responsive means or fuse will not blow to interrupt the faulted circuit to preclude progressive deterioration of the condition of the switch and to stop the fire hazard inherent in the defect of the switch. This failure to provide protection is due to the fact that a fuse is only responsive to overload conditions, and that the aforementioned danger condition is one generally concomitant with currents less than the rated current or the normal load current of the switch and the fuse.

It is, therefore, a further object of this invention to provide a switch-fusible-protective-device combination which effects automatic interruption of the electric circuit not only on occurrence of overloads therein but also on occurrence of any fault condition in the switch manifests itself by a dangerous increase in temperature.

. The foregoing and other general and special objects ing heavy contact of the invention and advantages thereof will become more apparent from the ensuing particular description thereof, as illustrated in the accompanying drawings, wherein Fig. 1 is substantially a side elevation, and in part a section along 1-1 of Fig. 2, of an enclosure housing a multipolar switch, of current-limiting back-up fuses for the switch, one associated with each pole thereof, and of additional thermally responsive interrupting devices, one associated with each pole of the switch;

Fig. 2 shows the same structure as Fig. 1 in part in front view and in part sectioned along 22 of Fig. 1;

Fig. 3 is a longitudinal section of one of the currentlimiting fuses shown in Figs. 1 and 2 taken along 3-3 of Fig. 4 showing the construction of such a fuse;

Fig. 4 is a section along 4-4 of Fig. 3;

Fig. 5 is in substance a longitudinal section of one of the additional thermally responsive interrupting devices shown in Figs. 1 and 2 in contact closed position;

Fig. 6 is a section along 6-6 of Fig. l; and

Fig. 7 shOWs on a larger scale in section a detail of Fig. 1.

Referring now to the drawings, and more particularly Figs. 1 and 2 thereof, reference numeral 1 has been applied to indicate a cabinet of steel housing a non-automatic manually operable switch generally indicated by numeral 2, current-limiting fuses generally indicated by numeral 3 and additional thermally responsive interrupting devices generally indicated by numeral 4. Cabinet 1 has a cover (not shown in the drawing) by which the cabinet can be closed when access to parts arranged therein is not required. Switch 2 is a multipolar switch intended for control of multiphase electric circuits. Only some of the poles of switch 2, i. e. two poles thereof, have been shown in Fig. 2. Switch 2 comprises an upper angular line contact 5a and a lower angular load contact 5. A barrier 7 of insulating material is arranged in the gap 6 formed between contacts 5a and 5. The contacts or jaws 5a, 5 support and guide a movable arcing chamber 8 made of a suitable insulating material, e. g. a plastic. Arcing chamber 8 is adapted to be manually operated by a lever 9 hingedly connected to the upper front edge thereof (see Fig. l) and pivotally mounted at 12 in cabinet 1. Barrier 7, the blade or slug contact 10 and the steel spring 11 are supported by, and adapted to be moved jointly with, the arcing chamber 8 from the off position shown in Fig. -l to the left to the on or closed position. This movement of parts 7, 8, 10 and 11 to the left is effected by counterclockwise rotation of lever 9 about pivot 12.

When lever 9 is turned in counterclockwise direction as seen in Fig. 1, barrier 7 is moved farther down into gap 6 and contact 10 enters into said gap, thus conductively connecting contacts 5a and 5. Upon continued movement of lever 9 in counterclockwise direction steel spring 11 engages the contacts or jaws 5a, 5, clamping them firmly against movable contact 10, thus providpressure and minimizing I -r losses in the switch.

-When lever 9 is turned in clockwise direction, steel spring 11 comes off contact jaws 5a, 5, thus reducing contact pressure preparatory to separating contact 10 from contacts 5a and 5 and to interrupting the circuit. The movable contact 10 moves out of gap 6 and out of engagement with stationary contact jaws 5a, 5 after the steel spring 11 has parted from the latter.

The fixed contacts 5a, 5 are mounted on a block 13 of insulating material secured to the back of cabinet 1. Block 13 supports U-shaped fuse clips 14 for receiving the lower blade contacts 40 of current-limiting fuses 3. Fuse clips 14 are directly mounted on the vertical sides of contacts 5a. Another insulating block 15 accommodated within cabinet 1 and secured to the back thereof supports fuse clips 16 receiving the upper blade contacts 40 of current-limiting fuses 3. Insulating block 15 supports also connectors 19 for cables 18 of the line or supply side of the circuit. A substantially L- shaped contact bar 17 is secured to the load contact of each pole of switch 2. One side of each L-shaped contact bar 17 is screwed to each contact 5, the other side thereof being arranged at right angles to the plane defined by insulating barrier 7 and contact 10.

Each of the additional interrupting devices 4- comprises a pair of terminal elements 18 which are substantially blade shaped. The upper terminal elements 13 are screwed to- L-shapedcontact bars 17, whereas the lower terminal elements 13 support connectors 19 to which the load cables 20 are attached. 7

The connectors 19 for line cables 18' and the connectors 19 for the load cables 20 are of the same type. Fig. 7 shows a connector 19 for a line cable 18 on a larger scale. The connector comprises a cylindrical metalblock 1% having a screw-threaded horizontal bore 19d for the insertion of a screw-threaded plug 191). The latter is provided with a recess 19c in the shape of a hexagon adapted for insertion of an appropriate key into it. Block 19:: has an additional bore 1% arranged at right angles to the plug-receiving bore 19d. This additional bore 192 is intended for the insertion of the stripped or bare ends of cables 18, as clearly shown in Figs. 1 and 2.

The current-limiting fuse illustrated in Figs. 3 and 4 comprises a pair of cylindrical terminal blocks 35 having systems of radial grooves 36 in the juxtaposed surfaces 37 thereof. Blocks 35 are conductively interconnected by multiperforated fuse links 38 of silver or copper which are inserted with the ends thereof into radial grooves 36 provided in surfaces 37 and conductively connected to blocks 35 by soft solder. All the links 33 are enclosed in a tubular casing 39 of heat-shock-resistant insulating material, preferably a synthetic resin-glasscloth laminate, mounted on blocks 35 and secured to blocks 35 by transverse steel pins 39. A pulverulent arc-quenching filler 41, preferably quartz sand from which metallic impurities have been carefully removed, occupies the space defined by casing 39 and surrounds the fuse links 38. Connectors in the form of blade contacts 40 adapting the structure for insertion into fuse clips 14, 16 (see Figs. 1 and 2) project in opposite directions from blocks 35. The aggregate cross-sectional area of links 38 is large enough to provide for current-carrying capacity far in excess of that of switch 2, and concomitant drastic reduction of 1 -1 losses far below the watt/ amp. level of a fuse designed for overload protection or combined overload and short-circuit current protection. -Yet the aggregate minimum cross-sectional area of links 38 is sufficiently small to effect'fusion and arc-initiation at the occurrence of: major fault currents before the fault current can reach the potential. or available peak value thereof.

The'structure ofthe, additional interrupting devices 4 preferably embodies the teachings of United States Patent 2,678,980 to Paul C. Hitchcock, Thermal Protection of Cables, May 18, 1954. Fig. 5 is substantially a longitudinal section of that structure in contact closed position.

Referring now to Fig. 5, reference numerals 21 and 22 have been applied to indicate a pair of coaxially arranged spaced tubular conductors. The axially outer ends of conductors 21 and 22 are compressed or squashed to form blade contacts 18'. If desired, only one of the tubular conductors 21, 22 is converted at the axially outer end thereof by squashing into a blade contact, i. e. the conductor intended to be connected to the L-shaped contact bar 17. The other tubular conductor may directly receive the bare or stripped end of one of the load cables 20 and be secured to it by an indenting tool capable of producing a very high pressure to establish a'flrm mechanical connection and a highly conductive bond between the respective conductor and the load cable connected to it. The adjacent ends of conductors 21, 22 are coated with a suitable soft low melting point solder. These solder coatings indicated by reference numeral 23 form a conductive connection between conductor 21 and tubular current-bridge 24 and between current-bridge 24 and conductor 22. Solder joints 23 normally restrain current-bridge 24 in the bridging position thereof, i. e. in the position shown in Fig. 5 wherein conductors 21, 22 are conductively interconnected by bridge 24. Both solder joints 23 are preferably cylindrical in the form of a fusible film filling the narrow gaps formed between conductor 21 and bridge 24 and between bridge 24 and conductor 22, respectively. Tubular insulating element 25 is slidably mounted on conductor 22 in abutting relation with current-bridge 24. Helical spring 26 is likewise mounted on conductor 22 and biases both: bridge 24 and insulating element or follower 25 upwardly away from the position shown in Fig. 5. The length of the tubular insulating element 25 exceeds the spacing between the juxtaposed ends of conductors 21, 22 so as to fill or bridge the gap formed between said juxtaposed ends when the solder joints 23 normally restraining bridge 24 become sufliciently soft to yield to the pressure of helical spring 26. Tubular insulating spacer 27 is pressed into the juxtaposed ends of conductors 21, 22 to maintain the proper spacing between these two parts. Spacer 27 may be provided with a pair of metal caps 28 closing spacer 27 and in electrical contact with conductors 21, 22. A multiperforated fusible link 29, preferably of silver or copper, arranged within spacer 29 interconnects conductively the caps 28. A pulverulent arcquenching filler 30 is arranged within spacer 27 and surrounds fusible link 29.

Bridge 24 shunts normally the currer -path comprising conductor 21, cap 28 in contact with conductor 21, link 29, cap 28 in contact with conductor 22 and conductor 22. 'Since the resistance of current-bridge 24 is much less than that of link 29, normally all the current carried by conductors 21, 22 flows through shunt path 23, 24, 23 and virtually none of it through link 29. On occurrence of overloads of excessive duration endangering the insulation of cables 20, or causing evolution of smoke therefrom, solder joints 23 become sufficiently heated by the 1 -1 losses in the cable to yield to the action of spring 26. As a result, bridge 24 andfoh lower 25' are being jointly moved by spring 26 from their normal position shown in Fig. 5 upwardly to the off position thereof. No arcing occurs as current-bridge 24 parts'from the points where it had been soldered to connectors 21, 22 since the potential of both connectors is equal at this moment by virtue of the presence of link 29. At the instant the shunt path through bridge 24 is being: interrupted by displacement of bridge 24 by the action of'spring'26, link 29 begins to carry current; but link 29 is wholly inadequate to carry currents in the order whichcan'continually be carried by bridge 24. Therefore link 29' fuses and vaporizes almost instantly upon interruption of the shunt path 23, 24, 23. Fusion of link 29 results in the formation of an arc; yet this arc is safely confined within the pressure-resistant tubular spacer 27 and rapidly extinguished by the heat-dissipating action of the pulverulent arc-quenching filler 30.

Each of the above additional interrupting devices 4 comprises a tubular casing 31 of insulating material. Casings 31- are supported by pairs of collars 32 mounted on conductors 21, 22. Transverse steel pins 33 establish afirm bond between casings 31 and collars 32.

The current-paths of the additional interrupting devices 4 have a relatively large cross-sectional area, i. e. they are designed to minimize the generation of heat therein. If but one single interrupting device 4 is provided per pole of switch 2, the resistance of the current-path of each interrupting device 4 is in the order of an equal'length of electric cable having the same current-rating as switch 2. Ifas shown in Fig. 1-a plurality of n parallel connected interrupting devices 4 is applied per pole of switch 2, the resistance of the current-path of each interrupting device is in the order of an equal length of an electric cable having l/n of the current rating of switch 2.

The yielding point of solder joints 23 is slightly below the critical temperature which, if continuously maintained, would endanger the insulation of the cables 20 on the load side of the unit. The electrical and thermal characteristics of conductors 21, 22 and of current-bridge 24 are so correlated that the temperature at the points where solder joints 23 are situated does normally not exceed the temperature of load cable connectors 19 or the bare ends of cables 20 therein. How such a correlation can be achieved has been shown in detail in United States Patent 2,645,690 to William S. Edsall et al., Thermal Protector for Conductor Insulation, July 14, 1953. Such correlation requires that the interrupting devices 4 be controlled substantially only by the heat generated in cables 20, i. e. be substantially unaffected by heat generated by any source of heat other than the cores of cables 20.

To minimize heat generation within cabinet or housing 1, the current-limiting fuses 3 which are arranged therein have a current-rating which is a multiple of the currentrating of switch 1. Current-limiting fuses having such a current-rating generate normally very little heat since they are normally very much underloaded. They are, however, capable of providing protection against major fault currents, i. e. currents of short-circuit current proportions. On the other hand, such fuses cannot provide protection against the dangerous thermal effects of relatively small but protracted overload currents on the insulation of cables 20. Since the additional interrupting device 4 are provided to effect thermal protection of the insulation of cables 20, the current-limiting fuses 3 are not required to provide this kind of protection and can be overrated fuses, as aforementioned.

Normally switch 2 generates but small amounts of heat, insuflicient to affect the proper operation of interrupting devices 4. When the contacts or contact members 5a, 5, of switch 2 become defective and overheat, solder joints 23 of interrupting devices 4 may be heated to the yielding point thereof by heat generated in switch 2 though the temperature-time-integral to the action of which the organic insulation of the load cables 20 is subjected is not sufficient to impair its insulating properties, or its life. Thus the circuit will be interrupted by .devices 4 on the occurrence of serious defects of the contacts 5a, 5 and 10 of switch 2 irrespective of the magnitude of current flow. This is as it should be, and the structure shown in Figs. 1 and 2 has been devised with the intention to facilitate heat transfer from contacts 5a, 5, 10 to solder joints 23 in devices 4. To this end the upper blade contacts 18 of devices 4 are arranged in the spaces bounded by the angular contacts 5 of switch 2, thus being effectively heated by radiation as well as convection through the intermediary of L-shaped contact bars 17.

If one of the additional interrupting devices 4 should not be properly connected into the circuit, resulting in undue generation of heat at the surfaces of blade contacts 18, such condition has the same effect as undue generation of heat at the contact members 5a, 10, 5 of switch 2, i. e. interruption of the circuit at temperatures and currents below those which would endanger the insulation of load cables 20. Only that interrupting device 4 will interrupt whose blades 18 are unduly heated by virtue of a faulty contact, whereas the interrupting devices 4 whose connectors 18 are properly inserted into the circuit maintain continuity of service in the healthy branches of the circuit.

The current-path of one pole of the device is as follows: line cable 18', clamp 17, fuse clip 16, current-limiting fuse 3 including blade contacts 40 thereof, fuse clip 14, switch 2 including contacts 5a, 10, and 5 thereof, L-

open position thereof and held in closed shaped contact bar 17, additional interrupting device 4 including blade contacts 18 thereof, clamp 19, load cable 20. I

By virtue of the presence of current-limiting fuses 3 switch 2 can safely be closed against any short circuit, since the current-limiting fuses 3 reduce inrush currents to safe peak values.

On acount of the facts that the current-limiting fuses 3 have a current-carrying ability far in excess of that of switch 2 and that the interrupting devices 4 are of the non-heat-genera-ting type, the temperature in cabinet 1 tends to be low and switch 2 does not need to be derated. On the other hand, interrupting devices 4 respond to excess temperatures irrespective of the magnitude of the current and thus provide protection against fault currents less than the rated current of switch 2.

Having fully described my invention and illustrated a preferred embodiment thereof it will be understood that various changes and modifications may be made in the combined structures by those versed in the art without departing from the spirit and scope of my invention; hence I do not intend to be limited by the exemplary preferred embodiment illustrated, but what I claim as new and wish to secure by Letters Patent is:

l. A switch and protective unit comprising a metal enclosure jointly housing a non-automatic manually operable switch having a predetermined current-rating, a current-limiting fuse having a current-rating substantially in excess of said predetermined current-rating adapted to be connected into an electric circuit in series with said switch, and an additional interrupting device adapted to be connected into an electric circuit in series with said switch and said fuse, said additional interrupting device including a pair of separable contacts normally spring biased to the open position thereof and held in closed position by soft solder conductively interconnecting said pair of contacts, the current-path of said additional interrupting device being designed to minimize generation of heat therein when carrying an electric current, and said additional interrupting device being arranged immediately adjacent said switch to be heated by excessive heat generated therein.

2. A switch and protective unit comprising a metal enclosure jointly housing a non-automatic manually operable switch having a predetermined current-rating, a current-limiting fuse having a current-rating which is a plural of said predetermined current-rating adapted to be connected into an electric circuit in series with said switch, and a plurality of additional interrupting devices adapted to be connected into an electric circuit in series with said switch and said fuse and parallel to each other, each of said plurality of interrupting devices including a pair of separable contacts normally spring biased to the open position thereof and held in closed position by soft solder conductively interconnecting said pair of contacts, the current-path of each said plurality of additional interrupting devices being designed to minimize generation of heat therein when carrying an electric current, and said plurality of interrupting devices being arranged immediately adjacent said switch to be heated by excessive heat generated therein.

3. A switch and protective unit comprising in combination a non-automatic manually operable switch including cooperating relatively movable contact members having a predetermined current-carrying capacity, a current-limiting fuse having a current-carrying capacity which is a multiple of said predetermined current-carrying capacity adapted to be connected into an electric circuit in series with said contact members, and an additional interrupting device adapted to be connected into an electric circuit in series with said contact members and said current-limiting fuse, said additional interrupting device including a pair of separable contacts normally spring biased to the position by solder means conductively interconnecting said pair of contacts,

the resistance of the current-path of said interrupting do vice being in the order of that of an-equal length of 'electric cable having said predetermined current-carrying capacity and said interrupting device being arrangedimmediately adjacent said 'switchto be heatedby excessive heat generated therein.

4. In combination an electric circuit, a switch and protective unit comprising in combination 'a non-automatic manually operable switch including cooperating relatively movable contact members having a predetermined 'current-carrying capacity and a current-limiting fuse having a current-carrying capacity which is a multiple of said predetermined current-carrying capacity, said fuse being connected into said electric circuit in series with. said'contact members, a plurality of n cable's each having 'l/n of said predetermined current-carrying capacity, and a plurality of 11 additional interrupting devices adapted to be connected into an electric circuit in series with said contact members and said current-limiting fuse and in parallel with respect to each other, each of said plurality of interrupting devices being connected to one of said plurality of cables and including a pair of separable contacts normally spring biased to the open position thereof and held in closed position by solder means conductively interconnecting said pair of contacts, the resistance of the current path of each of said plurality of interrupting devices being in the order of that of an equal length of one of said plurality of cables, and each of said plurality of additional interrupting devices-being arranged immediately adjacent said switch to be heated by excessive heat generated therein.

5.. A switch and protective unit comprising a first terminal clamp adapted to receive a line cable, a second terminal clamp adapted to receive a load cable, a non-automatic manually operable switch having a predetermined current-rating interposed between said first clamp and said second clamp, a current-limiting fusehaving a current-rating being a plural of said predetermined current-- rating interposed between said first clamp and said switch, and an additional interrupting device interposed between said switch and said second clamp, said additional interrupting device including a pair of separable contacts normally biased to the open position thereof and held in closed position by solder means conductively interconnecting said pair of contacts, and said interrupting device comprising a current-path of low resistance generating heat in the same order when carrying an electric current as a portion of a cable of equal length having said predetermined current-rating.

6. A switch and protective unit comprising in combination a line cable, a load cable, a non-automatic manually operable switch having a predetermined current-rating interposed between said line cable and said load'cable, a current-limiting fusehaving a current-rating which is a plural of said predetermined current-rating interposed between said line cable and said switch, and an additional thermally responsive interrupting device interposed between said switch and said load cable, said interrupting device including a pair of separable contacts normally spring-biased to the open position thereof and held in closed position by solder means conductively interconnecting said pair of contacts, and the current-path of said additional interrupting device having an ohmic resistance of the same order as the ohmic resistance of an equal length of said load cable.

7. A switch and protective unit comprising in combination a line cable, a plurality of :1 load cables, a non-automatic manually operable switch including a pair of cooperating relatively movable contact members having a predetermined current-carrying capacity interposed between said line cable and said plurality of load cables, a current-limiting fuse having a current-rating being a plural of said predetermined current-rating interposed between said line cable and said switch, and a plurality of n additional thermally responsive interrupting devices each interposed between said switch and one of said plurality of load cables, each of "said plurality of interrupting dc-- resistance of the same order as the ohmic resistance of an equal length of one of said plurality of load cables.

8. In combination an electric circuit, a switch and protective unit comprising in combination a non-automatic manually operable'switch having a predetermined current-rating and a-current-lirniting fuse having a currentratingin excess of said predetermined current-rating, said fuse being connected into said electric circuit in series with said switch, and a plurality of additional interrupting devices adapted to be connected into said electric circuit in series with said switch'and said fuse and parallel to each other, each of said plurality of additional interrupting devices including a pair of separable contacts normally spring biased to the open position thereof and held in closed position. by solder means conductively interconnecting. said "pair of contacts, the current-path of each of 'said plurality of interrupting devices being designed to minimize the generation of heat. therein, and each of said plurality ofinterrupting devices being arranged immediately adjacent said switch to be heated by heat generated therein.

9; A switch and protective unit comprising in combination'a non-automatic manually operable switch having a predetermined current-rating, said switch including a substantially angular line contact, a spaced substantially angular load contact, and a movable bridge contact adapted to be moved selectively into and removed from the gap..formed between said line contact and said load contact, a current-limiting fuse having a current-rating being a multiple of said predetermined current-rating arranged at the side of said switch adjacent said line contact and conductively connected to said line contact, and an additional interrupting device adapted to be connected into an electric circuit in series with said switch and said fuse, said interrupting device including a pair of separable contacts normally springbiased to the open position thereof and held inclosed position by solder means conductively interconnecting said pair of contacts, said interrupting device further comprising a casing accommodating said pair of contacts and a pair of connectors projecting from opposite sides of said casing, one of said pair of connectors beingarranged in the space bounded by and conductively connected to said lead contact to be heated fromvsaid switch jointly by conduction and radiation. of heat, and the. current-path of said interrupting device being designed to minimize the generation of heat therein.

10. A switch and protective unit comprising in combination-a non-automatic manually operable switch having a predetermined current-rating, said switch including a substantially angular line contact, a spaced substantially angular load contact, and a movable bridge contact adapted to be moved selectively into the gap and removed from the gap formed between said line contact and said load contact, said switch further including a substantially L-shaped contact bar secured with one side thereof to said load contact and arranged with the other side thereof at right angles to theplane defined by said bridge contact, a current-limiting fuse having a current-rating being a multiple of said predetermined current-rating arranged at the side of said switch adjacent said line contact, and an additional interrupting device adapted to be connected into an electric circuit in series with said switch and said fuse, said interrupting device including a pair of separable contacts normally spring biased to the open position thereof andheld, in closed position by solder means conduci tively interconnecting said pair of contacts, said interrupting device further comprising a casing accommodating said pair of contacts and a pair of connectors projecting from opposite sides of said casing, one of said pair of connectors being arranged in the space bounded by said load contact and firmly screwed against said L-shaped contact bar, and the current-path of said additional interrupting device being designed to minimize generation of heat therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,645,690 Edsall et al. July 14, 1953 10 Laing Oct. 27, 1953 Hitchcock Mar. 18, 1954 FOREIGN PATENTS Great Britain July 11, 1939 Great Britain Nov. 21, 1940 

